This invention relates to an improved synchronizing signal generator for a light beam scanner utilizing a rotary, multi-surfaced mirror, in which scanning direction jitter caused by mirror manufacturing errors, fluctuations in the mirror rotation, and mechanical vibrations is eliminated.
Devices for reading and recording information by using a laser light beam have been recently developed. In these devices a laser light scanning deflector is an essential element, which may take the form of an electro-optical or acousto-optical device, a vibrating galvanometer mirror, or a rotary, multi-surfaced mirror. The latter is particularly advantageous in that it has a large deflection angle and high resolution, and has no spectral characteristics. However, such a rotary, multi-surfaced mirror suffers from the drawback that the position of the start point of the laser scanning beam fluctuates or "jitters" because of angle division errors in manufacturing the mirror, variations in the rotational speed of the driving motor, etc, and the scanning is thus not always repeated at accurate time intervals.
To describe the angle division error, assume that a 24-surfaced mirror is employed and that 80% of the total scanning line width is utilized in the actual scan with a resolution of 1,500 dots or bits. The rotation angle of the mirror per dot is thus 360.degree./24.times.0.8.times.1/1500=0.008.degree.=28.8". Accordingly, if there is an error of more than 28.8" between adjacent mirror surfaces, a displacement of more than 1 dot is caused in the scanning direction between adjacent scan lines. Where the scanned image must have a high degree of resolution, a displacement of 1 dot destroys the image quality. The displacement must therefore be less than a fraction of 1 dot, and accordingly the angle division error must be less than several seconds. The manufacture of such a multi-surfaced mirror with high accuracy requires very sophisticated techniques, however, and is therefore very expensive.
With respect to the rotational speed of the motor, if a 24-surfaced mirror is revolved at 3,600 r.p.m., the scanning frequency is 1.44 KHz, and accordingly the period of time necessary for one scan line is 694.4 .mu.s. Since only 80% of the line width is utilized, the effective scanning time per dot or bit is 555.6 .mu.s/1,500=0.37 .mu.s. Rotational speed variations of the motor higher than 1 KHz are never encountered, and therefore the variation is one scan line, or during a period of 694.4 .mu.s, is so small that it can be neglected. However, any small speed variations are cumulative and result in a large low frequency variation from several Hz to several tens of Hz, and the measurement of this variation results in a time error of more than several .mu.s with respect to an ideally constant rotational speed. If such an error is produced, since the time necessary to scan a space corresponding to 1 dot is only about 0.37 .mu.s, a positional displacement of approximately fifteen or sixteen dots is caused with respect to the ideal image. Accordingly, in order to maintain the rotational speed variation at less than 1 dot, it is necessary to employ a high performance electric motor with an intricate feedback control.
Thus, there has been a strong demand to eliminate jitter in the scan line direction without improving the manufacturing accuracy of the multi-surfaced mirror or the rotational accuracy of the drive motor. To satisfy such demand, new methods are disclosed in Japanese Patent Publication (OPI) Nos. 40141/1976 and 89750/1976. In these methods a scan line is formed by deflecting a light beam with a rotary, multi-surfaced mirror, and a photo-detector is disposed in the light beam path in the vicinity of the start point of the line. The output of the photo-detector is employed as a synchronizing signal, and the light beam is modulated with information to be recorded a predetermined period of time after the production of the synchronizing signal, thereby starting the effective scan line at a predetermined position to eliminate jitter.
Such a method will be described with reference to FIG. 1, which is a schematic diagram illustrating a conventional optical system for generating synchronizing signals.
A light beam 100 applied to one surface 11 of a rotary mirror 10 is reflected in the direction of the arrow 14 as the mirror is rotated about the shaft 12 in the direction of the arrow 13. It is assumed that the deflected light beam is within an effective recording range when it is between a start beam 110 and a finish beam 120, and the beam thus provides an effective scan line 130 having a start point 111 and a finish point 121 on the film or image plane through a focusing lens 20. When the light beam is reflected by the mirror at a beam position 110', immediately before it starts its effective or recording deflection, the beam 110' passes through the lens 20 and forms a light spot 111' in the vicinity of the start point of the effective scan line, as shown in FIG. 1.
A photo-detector 30 is disposed in the path of the beam 110' at the position of the light spot 111' to thereby detect the light beam, and the output of the photo-detector is employed as a synchronizing signal. If, at a suitably predetermined time immediately after the end of the synchronizing signal the effective scan is started by modulating the light beam with information, the effective scan line starts at position 111 at all times, and jitter in the scanning direction is therefore eliminated.
The above-described method is somewhat effective, but still suffers from several drawbacks. In general, the light beam applied to the reflection mirror surfaces should have a finite width with respect to the direction of revolution. When the light beam is at the end of the deflection range, it is incident on the edge between adjacent mirror surfaces. If the deflection range is terminated before the light beam reaches the above-mentioned edge, then the effective scan width is decreased. If, when the light beam reaches the edge its power loss is less than 10-20%, this is generally regarded as the effective deflection range.
The use of an effective deflection range as wide as possible is strongly desirable where the diameter of the light beam incident on the mirror is increased to improve the image resolution or where the size of the mirror is reduced to increase the scanning rate. The light beam 110' applied to the photo-detector 30 after being reflected by the edge between adjacent mirror surfaces, and the light spot 111' formed thereby, are often irregular, however, because of the diffraction caused by the edge. This irregularity varies between adjacent mirror surface edges due to fluctuations in the edges caused by mirror manufacturing errors; that is, different irregularities appear for different mirror surface edges. Accordingly, synchronizing signals generated by the conventional apparatus shown in FIG. 1 tend to fluctuate with variations in the light beam 110' and the light spot 111', and the trailing edge of the output signal from the photo-detector 30 does not always correspond to the predetermined position of the beam 110' or the spot 111', which results in erroneous synchronizing signals.